Ultrafast proton release reaction and primary photochemistry of phycocyanobilin in solution observed with fs-time-resolved mid-IR and UV/Vis spectroscopy

Deactivation processes of photoexcited (lambda(ex) = 580 nm) phycocyanobilin (PCB) in methanol were investigated by means of UV/Vis and mid-IR femtosecond (fs) transient absorption (TA) as well as static fluorescence spectroscopy, supported by density-functional-theory calculations of three relevant ground state conformers, PCBA, PCBB and PCBC, their relative electronic state energies and normal mode vibrational analysis. UV/Vis fs-TA reveals time constants of 2.0, 18 and 67 ps, describing decay of PCBB*, of PCBA* and thermal re-equilibration of PCBA, PCBB and PCBC, respectively, in line with the model by Dietzek et al. (Chem Phys Lett 515:163, 2011) and predecessors. Significant substantiation and extension of this model is achieved first via mid-IR fs-TA, i.e. identification of molecular structures and their dynamics, with time constants of 2.6, 21 and 40 ps, respectively. Second, transient IR continuum absorption (CA) is observed in the region above 1755 cm(-1) (CA1) and between 1550 and 1450 cm(-1) (CA2), indicative for the IR absorption of highly polarizable protons in hydrogen bonding networks (X-(HY)-Y- horizontal ellipsis ). This allows to characterize chromophore protonation/deprotonation processes, associated with the electronic and structural dynamics, on a molecular level. The PCB photocycle is suggested to be closed via a long living (> 1 ns), PCBC-like (i.e. deprotonated), fluorescent species.

Automated summary

The deactivation processes of photoexcited phycocyanobilin (PCB) in methanol were studied using UV/Vis and mid-IR femtosecond transient absorption spectroscopy, supported by density-functional theory calculations. Significant insights into the molecular structures and dynamics were gained, allowing a better understanding of the chromophore protonation/deprotonation processes associated with electronic and structural dynamics. The PCB photocycle is proposed to be closed by a long living, fluorescent species resembling the PCBC conformer.